Operational filter building block

Abstract

A 3-terminal operational filter circuit is presented that can be used to construct various types of active filters. The filter circuit can be used to construct various orders of low pass, band-pass, high pass, and notch filters by simply coupling a few circuit elements (e.g., resistors and capacitors) to the filter circuit terminals. By cascading filter circuits, various types of higher order filters can constructed. The filter circuit can be manufactured with particular internal resistance and capacitance to enable the circuit to operate within a selected range of center frequencies. Users can then select a particular center frequency, notch frequency, quality factor, and gain of the filter circuit by selecting particular values for the circuit elements coupled to the filter circuit terminals.

I claim:  
     
       1. An electrical filter circuit operative to provide a notch frequency response comprising: 
       an operational filter circuit having an input terminal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal;  
       a first resistor coupled between said first output terminal and said input terminal;  
       a second resistor coupled between said input terminal and said second output terminal;  
       a third resistor coupled between an input signal source and said input terminal;  
       a summing circuit having a summing circuit input terminal, a summing circuit output terminal, and an operational amplifier having an input coupled to said summing circuit input terminal and an output coupled to said summing circuit output terminal;  
       a capacitor coupled between said second output terminal and said summing circuit input terminal; and  
       a fourth resistor coupled between said first output terminal and said summing circuit input terminal; wherein:  
       said summing circuit output terminal provides a notch frequency response.  
     
     
       2. The electrical filter circuit of claim  1  wherein said operational amplifier of said operational filter circuit has an inverting input coupled to said input terminal, a non-inverting input coupled to ground, and an amplifier output coupled to said second output terminal; said operational filter circuit further comprising: 
       a capacitor coupled between said inverting input and said amplifier output; and  
       a non-inverting integrator having an integrator input coupled to said amplifier output and an integrator output coupled to said first output terminal.  
     
     
       3. The electrical filter circuit of claim  1  wherein said summing circuit operational amplifier has an inverting input coupled to said summing circuit input terminal and a non-inverting input coupled to ground; said summing circuit further comprising: 
       a resistor coupled between said summing circuit input terminal and said summing circuit output terminal.  
     
     
       4. An electrical filter circuit operative to provide a notch frequency response comprising: 
       an operational filter circuit having an input terminal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal;  
       a first resistor coupled between said first output terminal and said input terminal;  
       a second resistor coupled between said input terminal and said second output terminal;  
       a third resistor coupled between an input signal source and said input terminal;  
       a capacitor coupled between said input signal source and said input terminal;  
       a summing circuit having a summing circuit input terminal, a summing circuit output terminal, and an operational amplifier having an input coupled to said summing circuit input terminal and an output coupled to said summing circuit output terminal;  
       a fourth resistor coupled between said second output terminal and said summing circuit input terminal; and  
       a fifth resistor coupled between said input signal source and said summing circuit input terminal; wherein:  
       said electrical filter circuit has a center frequency, and  
       said summing circuit output terminal provides a notch frequency response having a notch at a frequency greater than said center frequency.  
     
     
       5. The electrical filter circuit of claim  4  wherein said operational amplifier of said operational filter circuit has an inverting input coupled to said input terminal, a non-inverting input coupled to ground, and an amplifier output coupled to said second output terminal; said operational filter circuit further comprising: 
       a capacitor coupled between said inverting input and said amplifier output; and  
       a non-inverting integrator having an integrator input coupled to said amplifier output and an integrator output coupled to said first output terminal.  
     
     
       6. The electrical filter circuit of claim  4  wherein said summing circuit operational amplifier has an inverting input coupled to said summing circuit input terminal and a non-inverting input coupled to ground; said summing circuit further comprising: 
       a resistor coupled between said summing circuit input terminal and said summing circuit output terminal.  
     
     
       7. An electrical filter circuit operative to provide a notch frequency response comprising: 
       an operational filter circuit having an input terminal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal;  
       a first resistor coupled between said first output terminal and said input terminal;  
       a second resistor coupled between said input terminal and said second output terminal;  
       a third resistor coupled between an input signal source and said input terminal;  
       a capacitor coupled between said input signal source and said input terminal;  
       a summing circuit having a summing circuit input terminal, a summing circuit output terminal, and an operational amplifier having an input coupled to said summing circuit input terminal and an output coupled to said summing circuit output terminal;  
       a fourth resistor coupled between said first output terminal and said summing circuit input terminal; and  
       a fifth resistor coupled between said input signal source and said summing circuit input terminal; wherein:  
       said electrical filter circuit has a center frequency, and  
       said summing circuit output terminal provides a notch frequency response having a notch at a frequency less than said center frequency.  
     
     
       8. The electrical filter circuit of claim  7  wherein said operational amplifier of said operational filter circuit has an inverting input coupled to said input terminal, a non-inverting input coupled to ground, and an amplifier output coupled to said second output terminal; said operational filter circuit further comprising: 
       a capacitor coupled between said inverting input and said amplifier output; and  
       a non-inverting integrator having an integrator input coupled to said amplifier output and an integrator output coupled to said first output terminal.  
     
     
       9. The electrical filter circuit of claim  7  wherein said summing circuit operational amplifier has an inverting input coupled to said summing circuit input terminal and a non-inverting input coupled to ground; said summing circuit further comprising: 
       a resistor coupled between said summing circuit input terminal and said summing circuit output terminal.  
     
     
       10. A method of constructing an electrical filter operative to provide a notch frequency response, said method comprising: 
       providing an operational filter circuit having an input terminal for receiving an input signal from an input signal source, a first output terminal for providing a low Pass frequency response, and a second output terminal for providing a band-pass frequency response;  
       providing a summing circuit having a summing circuit input terminal and a summing circuit output terminal;  
       coupling a capacitor in series between said second output terminal and said summing circuit input terminal; and  
       coupling a resistor in series between said first output terminal and said summing circuit input terminal; wherein:  
       said summing circuit output terminal provides a notch frequency response.  
     
     
       11. A method of constructing an electrical filter operative to provide a notch frequency response, said method comprising: 
       providing an operational filter circuit having an input terminal for receiving an input signal from an input signal source, a first output terminal for providing a low pass frequency response, and a second output terminal for providing a band-pass frequency response;  
       providing a summing circuit having a summing circuit input terminal and a summing circuit output terminal;  
       coupling a resistor in series between said second output terminal and said summing circuit input terminal; and  
       coupling a second resistor in series between said input signal source and said summing circuit input terminal; wherein:  
       said electrical filter has a center frequency, and said summing circuit output terminal provides a notch frequency response having a notch at a frequency greater than said center frequency.  
     
     
       12. A method of constructing an electrical filter operative to provide a notch frequency response, said method comprising: 
       providing an operational filter circuit having an input terminal for receiving an input signal from an input signal source, a first output terminal for providing a low Pass frequency response, and a second output terminal for providing a band-pass frequency response;  
       providing a summing circuit having a summing circuit input terminal and a summing circuit output terminal;  
       coupling a resistor in series between said first output terminal and said summing circuit input terminal; and  
       coupling a second resistor in series between said input signal source and said summing circuit input terminal; wherein:  
       said electrical filter has a center frequency, and  
       said summing circuit output terminal provides a notch frequency response having a notch at a frequency less than said center frequency.  
     
     
       13. The electrical filter circuit of claim  1  wherein said summing circuit comprises a second operational filter circuit having an input terminal for receiving an input signal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal; wherein: 
       said summing circuit input terminal is said input terminal of said second operational filter circuit,  
       said summing circuit output terminal is said first output terminal of said second operational filter circuit, and  
       said summing circuit operational amplifier is said operational amplifier of said second operational filter circuit; said summing circuit further comprising:  
       a first summing circuit resistor coupled between said summing circuit output terminal and said summing circuit input terminal, and  
       a second summing circuit resistor coupled between said summing circuit input terminal and said second output terminal of said second operational filter circuit.  
     
     
       14. The electrical filter circuit of claim  1  wherein said summing circuit comprises a second operational filter circuit having an input terminal for receiving an input signal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal; wherein: 
       said summing circuit input terminal is said input terminal of said second operational filter circuit,  
       said summing circuit output terminal is said second output terminal of said second operational filter circuit, and  
       said summing circuit operational amplifier is said operational amplifier of said second operational filter circuit; said summing circuit further comprising:  
       a first summing circuit resistor coupled between said summing circuit output terminal and said summing circuit input terminal, and  
       a second summing circuit resistor coupled between said summing circuit input terminal and said first output terminal of said second operational filter circuit.  
     
     
       15. The electrical filter circuit of claim  4  wherein said summing circuit comprises a second operational filter circuit having an input terminal for receiving an input signal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal; wherein: 
       said summing circuit input terminal is said input terminal of said second operational filter circuit,  
       said summing circuit output terminal is said first output terminal of said second operational filter circuit, and  
       said summing circuit operational amplifier is said operational amplifier of said second operational filter circuit; said summing circuit further comprising:  
       a first summing circuit resistor coupled between said summing circuit output terminal and said summing circuit input terminal, and  
       a second summing circuit resistor coupled between said summing circuit input terminal and said second output terminal of said second operational filter circuit.  
     
     
       16. The electrical filter circuit of claim  4  wherein said summing circuit comprises a second operational filter circuit having an input terminal for receiving an input signal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal; wherein: 
       said summing circuit input terminal is said input terminal of said second operational filter circuit,  
       said summing circuit output terminal is said second output terminal of said second operational filter circuit, and  
       said summing circuit operational amplifier is said operational amplifier of said second operational filter circuit; said summing circuit further comprising:  
       a first summing circuit resistor coupled between said summing circuit output terminal and said summing circuit input terminal, and  
       a second summing circuit resistor coupled between said summing circuit input terminal and said first output terminal of said second operational filter circuit.  
     
     
       17. The electrical filter circuit of claim  7  wherein said summing circuit comprises a second operational filter circuit having an input terminal for receiving an input signal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal; wherein: 
       said summing circuit input terminal is said input terminal of said second operational filter circuit,  
       said summing circuit output terminal is said first output terminal of said second operational filter circuit, and  
       said summing circuit operational amplifier is said operational amplifier of said second operational filter circuit; said summing circuit further comprising:  
       a first summing circuit resistor coupled between said summing circuit output terminal and said summing circuit input terminal, and  
       a second summing circuit resistor coupled between said summing circuit input terminal and said second output terminal of said second operational filter circuit.  
     
     
       18. The electrical filter circuit of claim  7  wherein said summing circuit comprises a second operational filter circuit having an input terminal for receiving an input signal, a first output terminal for providing a first frequency response, a second output terminal for providing a second frequency response, and an operational amplifier having an input coupled to said input terminal and an output coupled to said first output terminal and to said second output terminal; wherein: 
       said summing circuit input terminal is said input terminal of said second operational filter circuit,  
       said summing circuit output terminal is said second output terminal of said second operational filter circuit, and  
       said summing circuit operational amplifier is said operational amplifier of said second operational filter circuit; said summing circuit further comprising:  
       a first summing circuit resistor coupled between said summing circuit output terminal and said summing circuit input terminal, and  
       a second summing circuit resistor coupled between said summing circuit input terminal and said first output terminal of said second operational filter circuit.  
     
     
       19. A method of creating a notch frequency response, said method comprising: 
       filtering an input signal to obtain a band-pass frequency response;  
       filtering said input signal to obtain a low pass frequency response;  
       coupling said band-pass frequency response through a capacitor to a summing node;  
       coupling said low pass frequency response through a resistor to said summing node; and  
       summing said responses at said summing node to create said notch frequency response.  
     
     
       20. The method of claim  19  wherein said filtering of said input signal to obtain a low pass frequency response comprises deriving said low pass frequency response through signal integration of said band-pass frequency response. 
     
     
       21. The method of claim  19  wherein said band-pass and low pass frequency responses are second order responses. 
     
     
       22. A method of creating a notch frequency response, said method comprising: 
       phase shifting an input signal;  
       filtering said phase-shifted input signal to obtain a band-pass frequency response;  
       coupling said band-pass frequency response through a first resistor to a summing node;  
       coupling said input signal through a second resistor to said summing node; and  
       summing said band-pass frequency response and said input signal at said summing node to create said notch frequency response.  
     
     
       23. The method of claim  22  wherein said band-pass frequency response is a second order response. 
     
     
       24. A method of creating a notch frequency response, said method comprising: 
       phase shifting an input signal;  
       filtering said phase-shifted input signal to obtain a low pass frequency response;  
       coupling said low pass frequency response through a first resistor to a summing node;  
       coupling said input signal through a second resistor to said summing node; and  
       summing said low pass frequency response and said input signal at said summing node to create said notch frequency response.  
     
     
       25. The method of claim  24  wherein said filtering of said phase-shifted input signal to obtain a low pass frequency response comprises: 
       filtering said phase-shifted input signal to obtain a band-pass frequency response; and  
       integrating said band-pass frequency response to obtain a low pass frequency response.  
     
     
       26. The method of claim  24  wherein said low pass frequency response is a second order response.